Revert "replace is_unsigned"
Revert "replace is_unsigned"

This reverts commit 409d1bd4bbf307dc75e17bf6a8120db0ac95fadc.

stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math